alpha-dihydrotetrabenazine is a metabolite of tetrabenazine. Tetrabenazine is a drug for the symptomatic treatment of hyperkinetic movement disorder and is marketed under the trade names Nitoman in Canada and Xenazine in New Zealand and some parts of Europe, and is also available in the USA as an orphan drug. On August 15, 2008 the U.S. Food and Drug Administration (FDA) approved the use of tetrabenazine to treat chorea associated with Huntington's disease (HD), the first in the US. The compound has been known since the 1950s. (Wikipedia)